[gordon]

You are looking at a relationship between ply rating and allowable speed,and I don't think there is one.

The 40 / 50 will come from the maximum allowable speed for agricultural tyres in a given country - entirely independent of the ply rating.

Ply rating will normally set load though - a higher number of plies will carry more load - but as long as balance and weight is taken into account it should run up to any reasonable speed, although a thicker tyre will deal less well with heat build up.

As an example. DUKW tyres were 10 ply from the factory. In the UK they fitted 12 ply which were even heavier ( got that teeshirt ) but until the 10-ply were reproduced many DUKWs ran perfectly well on 8-ply Gama Goat tyres at the same maximum speed. I never heard of any issues from running them, but of course restored DUKWs would not be loaded like the originals.

So, legally you can run those tyres at 40 or 50 kph, but realistically I'd expect to be able to run them, duly balanced, at 40 or 50 mph. Of course if you run them at 50mph and you are involved in an accident of any sort, expect your insurance company just to walk away. A brand new quality agricultural tyre is probably a better bet than a 70 year old original, regardless of ply rating or original design speed.